1. Field of the Invention
The present invention relates to a crystal holding apparatus which is used for growing a single crystal by a CZ method, and more particularly to a crystal holding apparatus used for crystal holding and pulling, which mechanically holds and pulls up a grown crystal at a top thereof.
2. Description of the Prior Art
As a material for a silicon wafer used for manufacturing a semiconductor device, a silicon single crystal is often used which is manufactured by the CZ method. In manufacture of the silicon single crystal by the CZ method, as known, a seed crystal held by a bottom of a pulling shaft is immersed in a silicon melt formed in a crucible, and from this condition, the pulling shaft is lifted with rotation to thereby grow the silicon single crystal below the seed crystal.
The seed crystal is in the form of a narrow rod constituted by the silicon single crystal with a diameter on the order of 10 and few mm, and a top thereof is coupled to a seed holder, and a bottom thereof is immersed in the silicon melt. When such a seed crystal is immersed in a hot silicon melt, dislocation occurs due to thermal shock. For this reason, xe2x80x9cneckingxe2x80x9d is carried out which narrows a diameter of the seed crystal after immersion of the seed crystal in the silicon melt and maintains the state for a while to eliminate the dislocation of the crystal. A required diameter of a neck is under 5 mm in terms of eliminating the dislocation, and a desired diameter is under 3 mm.
A mainstream silicon single crystal manufactured by the CZ method has had a diameter of 8 inch and a weight around 100 kg. Recently, however, a single crystal has been further upsized, and test manufacture of a silicon single crystal with a diameter of 12 inch has started.
An increased diameter of the single crystal greatly increases a weight thereof, and the single crystal with the diameter of 12 inch reaches a weight of 200 to 300 kg. The highest concentration of the weight is on the neck which is positioned at the top of the single crystal and is the smallest diameter portion, but breaking strength of the silicon is 20 kg/mm2, so that the neck needs to have a diameter of more than 5 mm for surely holding a silicon single crystal even of 200 kg. Therefore, stable pulling of the single crystal of 12 inch is impossible in terms of crystal holding.
As an especially effective technique of resolving this contradiction, there is crystal holding and pulling which does not depend on a neck. In the techniques described in the Japanese Patent Publication No. 5-65477 specification, Japanese Patent Laid-Open No. 10-81582 specification, Japanese Patent Laid-Open No. 10-81583 specification, or the like, as shown in FIG. 7, a seed crystal 3 held by a seed chuck 3 is necked, then below a neck 11, a stopped portion 12 is formed which has a larger diameter than the neck 11 and is capable of mechanical holding, and a conventional body 13 is formed therebelow. When the stopped portion 12 is formed, a crystal holding apparatus 20 mechanically holds the stopped portion 12 to pull up a single crystal 10.
In order to prevent falling of the single crystal 10, the stopped portion 12 generally has a shape with a wide upper portion and narrow lower portion, and the narrow lower portion is set to have a sufficiently larger diameter than the neck. The wide upper portion is like a knob, and the lower portion is necked with respect to a body 13 therebelow, so that the stopped portion 12 is called a knob portion or necking portion.
A known crystal holding apparatus 20 has a cylindrical frame 21 which lifts and lowers and rotates for pulling up a single crystal 10 and which is circumferentially mounted with a plurality of pivot clamps 22, 22 at regular intervals. A pulling shaft 1 used for growing the single crystal 10 is inserted in the frame 21, and a seed chuck 2 coupled to a bottom of the pulling shaft 1 is pulled into the frame 21 with growth of the single crystal 10. A plurality of pivot clamps 22, 22 are brought into close condition by inward pivoting, grasp the stopped portion 12 of the single crystal 10 in the close condition, and are pivotable upward from the close condition.
General use of the crystal holding apparatus 20 using the pivot clamps 22, 22 will be as described below.
The crystal holding apparatus 20 brings the pivot clamps 22, 22 into the close condition and waits above the single crystal 10. When the single crystal 10 grows in this state, the seed chuck 2 first passes inside the pivot clamps 22, 22. The neck 11 of the single crystal 10 then passes, and the stopped portion 12 enters inside the pivot clamps 22 to be automatically grasped. Then, the frame 21 starts lifting and rotating synchronously with the pulling shaft. After that, the single crystal 10 is mechanically held and pulled up at the stopped portion 12 by the crystal holding apparatus 20. The pivot clamps 22, 22 are pivotable upward from the close condition and do not prevent passage of the seed chuck 2 and entry of the stopped portion 12.
As described above, the crystal holding apparatus 20 using the pivot clamps 22, 22 has a simple structure which requires no operation mechanism of the pivot clamps 22, 22, and for this advantage, the pivot clamps 22, 22 are generally set to the close condition from the beginning and not especially operated. However, it is also considered in some crystal holding apparatuses 20 that pivot clamps 22, 22 are set to an open condition, and after entry of a stopped portion 12 inside the pivot clamps 22, 22, a push rod is pushed up at a body 13 (shoulder) of a single crystal 10 to operate the pivot clamps 22, 22 to a close condition (Japanese Patent Laid-Open No. 10-81582 specification).
However, the conventional crystal holding apparatus 20 using the pivot clamps 22, 22 has the following problems.
The seed chuck 2 which holds the seed crystal 3 is formed, at an upper portion thereof, from metal such as molybdenum for coupling with the pulling shaft 1, but at a lower portion thereof which holds the seed crystal 3, from carbon for preventing metal contamination of the seed crystal 3. When the pivot clamps 22, 22 are not operated, that is, the pivot clamps 22, 22 are set to the close condition from the beginning, the pivot clamps 22, 22 rub a surface of the seed chuck 2 when the seed chuck 2 passes inside the pivot clamps 22, 22, thus there is a possibility that powdered carbon falls to interfere with growing of the single crystal 10.
Also, when the seed chuck 2 enters inside the pivot clamps 22, 22 or comes out therefrom, vibration occurs due to passage of steps, which also has a possibility of interfering growing of the single crystal 10.
When the pivot clamps 22, 22 are set to the open condition, and the pivot clamps 22, 22 are operated to the close condition after the entry of the stopped portion 12 inside the pivot clamps 22, 22, the seed chuck 2 passes inside the pivot clamps 22, 22 without any contact, thereby solving the above described various problems caused by the passage.
However, shifting operation from the open condition to close condition of the pivot clamps 22, 22 is carried out by pushing up the push rod at the body 13 (shoulder) of the single crystal 10, and the push rod needs to be located at a position abutting against the body 13 (shoulder), thereby causing a problem of radially upsizing the crystal holding apparatus 20.
The body 13 (shoulder) has a larger diameter and higher temperature than those of the stopped portion 12, thereby having a possibility of causing contamination by contact with the push rod.
The pivot clamps 22, 22 operated inward by the push rod falls inward by their own weights to collide against a surface of the stopped portion 12. There is also a possibility that this shock inhibits stable growing of the single crystal 10.
An object of the present invention is to provide a crystal holding apparatus which can solve a problem of carbon falling when a seed chuck passes inside pivot clamps even with a simple structure using the pivot clamps, and can avoid radial upsizing of the apparatus and contamination of a single crystal due to contact with the apparatus.
Another object of the present invention is to provide a crystal holding apparatus which can avoid both collision against the single crystal due to falling of the pivot clamps by their own weights and shock due to the falling per se.
In order to attain the above described objects, a crystal holding apparatus of the invention includes: a frame in which a pulling shaft for growing a single crystal is inserted and which lifts and lowers and rotates for pulling up a grown crystal; a plurality of pivot clamps which are pivotably mounted to a plurality of circumferential positions on the frame for gripping a top of the grown crystal; and a clamp operation mechanism which is incorporated in the frame so as to freely lift and lower, and is pushed up by a seed chuck coupled to a bottom of the pulling shaft to be lifted in the frame, and shifts the plurality of pivot clamps from an open condition to a close condition by the lift.
In the crystal holding apparatus according to the present invention, the plurality of pivot clamps are shifted from the open condition to close condition by the clamp operation mechanism, and by bringing the pivot clamps into the open condition from the beginning, the seed chuck passes inside the pivot clamps without contact. This permits avoiding contact between the pivot clamps and seed chuck. Further, the shift from the open condition to close condition is carried out by pushing up the seed chuck, and lift of the grown crystal is not used for operation of the pivot clamps, permitting avoiding contact between the grown crystal and apparatus.
The clamp operation mechanism preferably has a structure in which the plurality of pivot clamps are shifted from the open condition to close condition when a neck of the grown crystal passes inside the pivot clamps. The pivot clamps make no contact with the narrow neck even in the close condition, so that collision of the pivot clamps against the grown crystal is avoided by shifting the pivot clamps from the open condition to close condition in passage of the neck. The pivot clamps shifted to the close condition then gently pivot along a smooth outer shape of the top of the crystal lifted inside to grip a holding position.
As a specific configuration of the clamp operation mechanism, it is preferable that the plurality of pivot clamps have tapers inclined outward in an upward direction on clamp surfaces which face outward when the respective pivot clamps are in the open condition, and that the clamp operation mechanism has an annular operation member which fits to outsides of the plurality of pivot clamps below the tapers and is pushed by the seed chuck to be lifted in the frame. According to this configuration, the plurality of pivot clamps are synchronously shifted from the open condition to close condition with the simple structure.
The clamp operation mechanism preferably has a configuration in which, when the respective pivot clamps are about to fall in a close direction by their own weights, the falling is inhibited and the respective pivot clamps are rotated in the close direction. According to this configuration, the shift from the open condition to close condition is gradually carried out synchronously with the lift of the seed chuck, thereby preventing shock due to the falling per se of the pivot clamps.
As a specific configuration for inhibiting the falling of the pivot clamps, it is preferable that the plurality of pivot clamps are provided with projections on the clamp surfaces which face outward when the respective pivot clamps are in the open condition, the projections being provided with recesses facing outward and slits for opening the recesses upward, and that the clamp operation mechanism engages a bottom of a downward operation member pushed by the seed chuck to be lifted in the frame with the recesses via the slits.
According to this specific configuration, while the centers of gravity of the pivot clamps are outside the pivots, the pivot clamps are forcedly operated in the close direction with the projections pulled up, and after the centers of gravity of the pivot clamps move inside the pivots, the projections are pushed down to inhibit inward falling of the pivot clamps by their own weights, and the operation member is pulled up from the recesses to release the pivot clamps in the close condition.
The clamp operation mechanism preferably has a configuration in which the pivot clamps are pivotable in the close condition, so that after the shift from the open condition to close condition, the pivot clamps are released to allow passage of the top of the crystal or the like.